For the Panzerzug train I also want to add a Leopold railway gun, and I've put together a basic model to add to the Sketchup model of the train. It's pretty long at around 34cm (the travelling mode of the real gun was 30m long, which scales to 344mm at 1:87 scale so the Sketchup model is pretty much spot on ), and may have trouble going around corners - although I might be able to mitigate the problems by using only 2 pairs of flanged wheels in each bogie, so that they act as a pair of long wheelbase railcars at each end supporting the gun carriage.

And a couple of "sketches" while messing with the Sketchup display options:

Technically not part of the BP42/BP44 trains as the Panzerjager-Triebwagens 50-55 never saw service, I wanted to include one of these Panzer IV turret armed railcars in the train model. This is the undetailed model to use as the basic plasticard template, with lots of hatches to be added during the build. The turrets are really rough but give an idea of how the final model will look.

If I can get my hands on a few 1:76 or 1:72 scale PzIV turrets from model kits it'll make life easier, but I don't really need to buy entire model kits - I'll need at least 4 PzIV turrets in total, 2 for this railcar and 2 more for the Panzerjagerwagens (flat cars with a single PzIV turret) that will be at each end of the train. I even looked at Plastic Soldier Company 1:72 PzIVs as a possible option but...

To make sure that the locomotive and tender won't look too big or too small alongside the existing OO scale trains that my son has on his railway a card mockup of the Sketchup model has been made. Using the Flattery plugin it was pretty easy to unfold the model faces to create a flat printable mesh that could then be folded and glued.

This is scaled at 1:87 (which is HO scale rather than OO, but means that the train will be in proportion to the track width), and next to a small 0-4-0 steam engine looks about the right size. This card mockup will also be used to find a chassis to fit the locomotive, and I've already got donor parts for the tender chassis from an old flat bed carriage which has 6 wheeled bogies at each end (the tender is sitting on one of these bogies in the photo below).

Started working on some of the smaller details on the locomotive and tender, adding buffers, armour plates for the buffers, boiler hatches, chimney, and driver cab windows.

As a mockup this works pretty well and allows me, using the Flattery plugin, to unfold the outer surfaces and create a mesh that I can print out - initially using a card template to check that the shells will fit the running gear and won't too look out of scale on the track, and then to cut out the main plasticard panels. Smaller details like inspection hatches and armour strips can be drawn in Photoshop and printed out later as they will be flat and simply cut out of thin plasticard and glued into place on the main surfaces. Here's the locomotive main shell flattened out:

Not much progress today, decided to add some track to the model and duplicate the tender (the real train had two tenders, one behind the locomotive and one in front), and add a rough camo texture to the armour plates. Once the chassis frame and buffers are added it will look a lot better, hopefully I'll progress with that tomorrow, and if there's time also add some of the larger hatches and the driver windows and vision slits.